1. Field of the Invention
The present invention relates to piezoelectric filters, and more particularly, to a piezoelectric filter in which two dual-mode filter elements are connected to each other in a face-to-face arrangement.
2. Description of the Related Art
In the related art, a piezoelectric filter in which two dual-mode filter elements are connected such that they face each other with a relay capacitance interposed therebetween is used as an intermediate frequency stage filter in mobile communication equipment such as portable telephones. Such a piezoelectric filter is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 10-284985.
The piezoelectric filter has first and second dual-mode filter elements which are stacked and bonded to each other. The first filter element has first and second major surfaces, with split electrodes on the first major surface and a common electrode, which faces the split electrodes, on the second major surface. Further, a capacitance electrode, which also functions as a terminal electrode, is disposed on the first major surface, and a capacitance electrode, which faces the capacitance electrode of the first major surface, is disposed on the second major surface to provide a capacitance portion. The second filter element also has a similar configuration, and one of the split electrodes of the first filter element and one of the split electrodes of the second filter element are interconnected. In addition, ground terminal electrodes of both filter elements are interconnected.
In the piezoelectric filter of the related art, unpolarized sections are provided at edges of the piezoelectric substrate of each filter element, and the terminal electrodes and capacitance electrode are provided within the unpolarized sections. Thus, this arrangement allows undesirable, unwanted vibration to be suppressed, thereby providing good filter characteristics.
In such a piezoelectric filter, however, the energy-trapping states of filter elements are equal. As a result, frequencies at which unwanted vibrations are generated are also the same, so that the unwanted vibrations are superimposed with each other, resulting in greater undesired vibration responses. To reduce such unwanted vibration responses, frequencies of respective filter elements can be displaced from each other. This approach, however, causes degradation in the attenuation characteristic of the piezoelectric filter, and thus is undesirable.
In order to overcome the problems described above, preferred embodiments of the present invention provide a piezoelectric filter which effectively eliminates unwanted vibration responses by displacing the frequencies of unwanted vibrations, without displacing the center frequencies of two filter elements.
According to a first preferred embodiment of the present invention, a piezoelectric filter includes first and second energy-trapping dual-mode filter elements. The first filter element has a first piezoelectric substrate having first and second major surfaces, split electrodes provided on the first major surface, a common electrode which is provided on the second major surface and which faces the split electrodes, and terminal electrodes provided at or adjacent to edges of both the first and the second major surfaces, each of the terminal electrodes being connected with one of the split electrodes and the common electrode. The second filter element has a second piezoelectric substrate having first and second major surfaces, split electrodes provided on the first major surface, a common electrode which is provided on the second major surface and which faces the split electrodes, and terminal electrodes provided at or adjacent to edges of both the first and the second major surfaces, each of the terminal electrodes being connected with one of the split electrodes and the common electrode. One of the split electrodes of the first filter element is interconnected with one of the split electrodes of the second filter element. Unpolarized sections are provided at edges of each of the first and second piezoelectric substrates, and the first and second filter elements each have a vibration region in which the energy-trapping vibration is generated, portions of each vibration region being located in the unpolarized sections. The ratio of the area of the portions of the vibration region which are located in the unpolarized sections of the first filter element to the area of the entire vibration region of the first filter element is different from the ratio of the area of the portions of the vibration region which are located in the unpolarized sections of the second filter element to the area of the entire vibration region of the second filter element.
With this arrangement, since the area ratio of the unpolarized vibration regions, which are the portions of the vibration region located in the unpolarized sections, of the first filter element is different from the area ratio of the unpolarized vibration regions of the second filter element, the energy-trapping states of the two filter elements are different from each other. Thus, frequencies at which unwanted vibrations are generated are different from each other, so that the responses of unwanted vibrations can be minimized and eliminated without unwanted vibrations of the two filter elements being superimposed. In addition, this arrangement can displace the center electrodes of the two filter elements, so that degradation of the attenuation characteristic of the piezoelectric filter is prevented.
As methods of varying the area ratios of the unpolarized vibration regions between two filter elements, for example, there are some approaches to provide filter elements having the following configurations (1) to (3):
(1) two filter elements having the same vibration regions (area, shape) and having different unpolarized sections (area, shape);
(2) two filter elements having the same unpolarized sections (area, shape) and having different vibration regions (area, shape); and
(3) two filter elements having different vibration regions (area, shape) and having different unpolarized sections (area, shape).
Preferably, the split electrodes on the first filter element are preferably arranged so as to oppose each other and the opposing direction thereof is tilted at an angle xcex81 with respect to an end surface of the first piezoelectric substrate. The split electrodes on the second filter element are preferably arranged so as to oppose each other and the opposing direction thereof is tilted at an angle xcex82 with respect to an end surface of the second piezoelectric substrate. The angle xcex81 and the angle xcex82 are different from each other. That is, as the methods (2) and (3) for achieving different vibration regions, the tilt angles xcex81 and xcex82 in the opposing direction of the split electrodes are configured to be different from each other. The different tilt angles xcex81 and xcex82 provide different frequencies at which the unwanted vibrations are generated from the two filter elements, and combining the different frequencies can reduce the unwanted vibrations. In this case, the unpolarized sections may or may not be the same.
Preferably, the first piezoelectric substrate and the second piezoelectric substrate have substantially the same outer shape, and the area occupied by the unpolarized section in the first piezoelectric substrate and the area occupied by the unpolarized section in the second piezoelectric substrate are different from each other. This arrangement uses the methods (1) and (3). Due to the two different unpolarized areas, energy-trapping states of the two filter elements are different from each other, which can suppress unwanted vibrations.
Other features, characteristics, elements and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.